1. Field of the Invention
The present invention relates to a MVA (Multi-domain Vertical Alignment) mode liquid crystal display device, and particularly to a liquid crystal display device in which a polymer for determining a direction in which liquid crystal molecules tilt while voltage is being applied is formed in a liquid crystal layer thereof.
2. Description of the Prior Art
In general, a liquid crystal display device is constituted of: a liquid crystal panel which is fabricated to contain liquid crystal between two substrates thereof; and polarizing plates which are arranged respectively in the two sides of the liquid crystal panel. A picture element electrode is formed in each of picture elements in one substrate of the liquid crystal panel. A common electrode used commonly for the picture elements is formed in the other substrate of the liquid crystal panel. When voltage is applied between the picture element electrode and the common electrode, alignment directions of liquid crystal molecules change depending on the voltage. As a result, this changes an amount of light which passes the liquid crystal panel and the polarizing plates arranged respectively on the two sides of the liquid crystal panel. If applied voltage were controlled for each of the picture elements, a desired image can be displayed on the liquid crystal display device.
With regard to a TN (Twisted Nematic) mode liquid crystal display device which has been heretofore used widely, liquid crystal with positive dielectric anisotropy is used, and liquid crystal molecule is twisted and aligned between the two substrates. However, the TN mode liquid crystal display device has a disadvantage of having insufficient viewing angle characteristics. In other words, with regard to the TN mode liquid crystal display device, tone and contrast are extremely deteriorated when the liquid crystal panel is viewed in an oblique direction. Accordingly, the contrast is reversed in extreme cases.
An IPS (In-Plane Switching) mode liquid crystal display device and a MVA (Multi-domain Vertical Alignment) mode liquid crystal display device have been known as liquid crystal display devices having good viewing angle characteristics. With regard to the IPS mode liquid crystal display device, picture element electrodes shaped like a line and common electrodes shaped like a line are arranged alternately in one of the two substrates. If voltage were applied between one of the picture element electrodes and neighboring one of the common electrodes, the orientations respectively of the liquid crystal molecules change in a plane parallel with a surface of the substrate depending on the voltage.
Although, however, the IPS mode liquid crystal display device is good at viewing angle characteristics, the orientations respectively of the liquid crystal molecules above the picture element electrode and the common electrode cannot be controlled since voltage is applied in a direction which is parallel with the substrate. This brings about a disadvantage that the IPS mode liquid crystal display device substantially has a low aperture ratio, and that the screen of it is dark if a powerful backlight were not used.
With regard to the MVA mode liquid crystal display device, picture element electrodes are formed in one of the two substrates, and a common electrode is formed in the other of the two substrates. In addition, with regard to a generally-used MVA mode liquid crystal display device, bank-shaped protrusions made of dielectric material extending in an oblique direction are formed on the common electrode. Each of the picture element electrodes is provided with slits parallel with the protrusions.
With regard to the MVA mode liquid crystal display device, while voltage is not being applied, the liquid crystal molecules are aligned in a direction perpendicular to the substrates. When voltage is applied between each of the picture element electrodes and the corresponding common electrode, the liquid crystal molecules are aligned to tilt at an angle corresponding to the voltage. In this occasion, a plurality of domains are formed in each of the picture elements by the slits provided into the picture element electrode and by the corresponding bank-shaped protrusions. The directions in which the liquid crystal molecules tilt vary from one domain to another. If the plurality of domains were formed in any one of the picture elements while the directions in which the liquid crystal molecules tilt vary from one domain to another, good viewing angle characteristics can be obtained.
With regard to the aforementioned MVA mode liquid crystal display device, the slits and the protrusions decrease the substantial aperture ratio. Accordingly, the substantial aperture ratio of the MVA mode liquid crystal display device is lower than that of the TN mode liquid crystal display device, although the substantial aperture ratio is not so low as that of the IPS mode liquid crystal display device. For this reason, the MVA mode liquid crystal display device needs a powerful backlight. As a result, this kind of MVA mode liquid crystal display device has hardly been adopted for a notebook personal computer, which requires power consumption to be low.
Japanese Patent Laid-open Official Gazette No. 2003-149647 has disclosed a MVA mode liquid crystal display device which was developed in order to solve the aforementioned problems. FIG. 1 is a plan view showing the MVA mode liquid crystal display device. Incidentally, FIG. 1 shows two picture element regions.
A plurality of gate bus lines 11 extending in the horizontal direction (X-axis direction) and a plurality of data bus lines 12 extending in the vertical direction (Y-axis direction) are formed on one of the two substrates constituting a liquid crystal panel. An insulating film (gate insulating film) is formed in each of the rectangular areas defined by the gate bus lines 11 and the data bus lines 12. This formation electrically isolates the gate bus lines 11 from the data bus lines 12. Each of the rectangular areas defined by the gate bus lines 11 and the data bus lines 12 is a picture element region.
A TFT (thin film transistor) 14 and a picture element electrode 15 are formed in each of the picture element region. As shown in FIG. 1, the TFT 14 uses part of the gate bus line 11 so as to cause the part to function as a gate electrode. A semiconductor film (not illustrated) which functions as an active layer of the TFT 14 is formed above the gate electrode. A drain electrode 14a and a source electrode 14b are connected respectively to the two sides of this semiconductor film in the Y-axis direction. The source electrode 14b of the TFT 14 is electrically connected to the data bus line 12, and the drain electrode 14a is electrically connected to the picture element electrode 15.
In this patent application, out of the two electrodes connected to the semiconductor film which functions as the active layer of the TFT, one electrode to be connected to the data bus line is termed as a source electrode, and the other electrode to be connected to the picture element electrode is termed as a drain electrode.
The picture element electrode 15 is formed of a transparent conductive material such as ITO (Indium-Tin Oxide). Slits 15a are formed in this picture element electrode 15 in order to cause liquid crystal molecules to be aligned in one of four directions when voltage is applied. In other words, the picture element electrode 15 is divided into four domains with the center line in parallel with the X-axis direction and the center line in parallel with the Y-axis direction defined as boundaries. A plurality of slits 15a extending in a direction at an angle of 45 degrees to the X axis are formed in a first domain (upper right domain). A plurality of slits 15a extending in a direction at an angle of 135 degrees to the X axis are formed in a second domain (upper left domain). A plurality of slits 15a extending in a direction at an angle of 225 degrees to the X axis are formed in a third domain (lower left domain). A plurality of slits 15a extending in a direction at an angle of 315 degrees to the X axis are formed in a fourth domain (lower right domain). A vertical alignment film (not illustrated) made of polyimide is formed on the picture element electrode 15.
Black matrices, color filters and a common electrode are formed in the other substrate. The black matrices are made of a metal such as Cr (chromium), or of black resin. The black matrices are arranged respectively in positions, each of which is opposite to any one of the gate bus lines 11, the data bus lines 12 and the TFTs 14. The color filters are classified into three types, such as red, green and blue. Any one of the three types of color filters is arranged in each of the picture elements. The common electrode is made of a transparent conductive material such as ITO, and is formed on the color filters. A vertical alignment film made of polyimide is formed on the common electrode.
A liquid crystal panel is constituted in the following manner. These substrates are arranged to be opposite to each other with spacers (not illustrated) interposed between the two substrates. Liquid crystal with negative dielectric anisotropy is filled between the two substrates. Hereinafter, out of the two substrates constituting the liquid crystal panel, one substrate on which a TFT is formed will be termed as a TFT substrate, and the other substrate which is arranged to be opposite to the TFT substrate will be termed as an opposing substrate.
In the case of the MVA mode liquid crystal display device shown in FIG. 1, the liquid crystal molecules are aligned virtually perpendicularly to the surface of each of the substrates while voltage is not being applied to the picture element electrode 15. When voltage is applied to the picture element electrode 15, the liquid crystal molecules 10 tilt in the directions in which the respective slits 15a extend as schematically shown in FIG. 1. Accordingly, four domains are formed in any of the picture elements while the directions in which the liquid crystal molecules tilt vary from one domain to another. This inhibits light from leaking in oblique directions, and thus securing good viewing angle characteristics.
Changing the subject. In the case of the MVA mode liquid crystal display device shown in FIG. 1, it remains to be determined whether the liquid crystal molecules 10 tilt inwards (in directions of the center of the picture element) or outwards (in directions of the outside of the picture element), immediately after voltage is applied to the picture element electrode 15. First of all, the electric field in extremities of the picture element electrode determines the liquid crystal molecules 10 in extremities of the slits 15a (near the data bus line 12) to tilt inwards. Subsequently, the liquid crystal molecules 10 in positions inwards from the extremities tilt towards the center of the picture element. Then, the liquid crystal molecules 10 in positions further inwards from the extremities tilt towards the center of the picture element. This process is repeated until all the liquid crystal molecules tilt towards the center of the picture element. Accordingly, it takes time for all the liquid crystal molecules 10 in a picture element to complete tilting in predetermined directions. This brings about a problem that the response time is long.
The aforementioned Japanese Patent Laid-open Official Gazette No. 2003-149647 has disclosed that a liquid crystal display device is fabricated in the following manner. First, liquid crystal to which a polymer component (monomer) is added is filled into the space between the pair of the substrates. Then, voltage is applied between the picture element electrode and the common electrode, thereby causing the liquid crystal to align in predetermined directions. Thereafter, beams of ultraviolet light are irradiated to the polymer component, and thereby the polymer component is polymerized. By this, polymer is made in the liquid crystal layer. In the case of the liquid crystal display device thus fabricated, the polymer in the liquid crystal layer determines directions in which the liquid crystal molecules tilt. For this reason, no sooner is voltage applied between the picture element electrode and the common electrode than all of the liquid crystal molecules in the picture element start to tilt in predetermined directions. Accordingly, the response time is reduced to a large extent.
In addition, addition of a polymer component to liquid crystal has been disclosed, also, by Japanese Patent Laid-open Official Gazette No. Hei. 11-95221 and Japanese Patent Laid-open Official Gazette No. Hei. 8-36186.
In general, in the case of a vertical alignment (VA) mode liquid crystal display device, it has been known that the gray-scale brightness characteristics to be observed when the liquid crystal display device is viewed from the front is different from that to be observed when the liquid crystal display device is viewed in an oblique direction. The aforementioned MVA mode liquid crystal display device also has the same defect. FIG. 2 is a diagram showing a gray-scale brightness characteristics to be observed when the MVA mode liquid crystal display device is viewed from the front, and a gray-scale brightness characteristics to be observed when the MVA mode liquid crystal display device is viewed in a direction at an azimuth angle of 90 degrees and at a polar angle of 60 degrees (in a direction downwards diagonally). In FIG. 2, the axis of abscissa represents the gray scale, and the axis of ordinate represents the transmittance. It should be noted that, in this patent application, the center of the liquid crystal panel is defined as the origin of ordinates, an angle between the x axis of the liquid crystal panel and a line along which a line of sight is projected onto the liquid crystal panel is termed as an azimuth angle, and an angle between a normal line of the liquid crystal panel and the line of sight is termed as a polar angle. Brightness between black and white is divided into 256 gray scales in FIG. 2. Each gray scale corresponds to applied voltage to a picture element electrode. The larger the gray scale number is, the larger voltage is applied to the picture element electrode. Furthermore, in FIG. 2, a transmittance is indicated by a value relative to the transmittance (Twhite) which is defined as 1 (one) when white is displayed.
As understood from FIG. 2, in the case of the conventional MVA mode liquid crystal display device, the gray-scale transmittance characteristics to be observed when the liquid crystal display device is viewed from the front is much different from that to be observed when the liquid crystal display device is viewed in an oblique direction. For this reason, the conventional MVA mode liquid crystal display device has a disadvantage that the display quality is deteriorated when viewed in an oblique direction although a preferable display quality can be obtained when viewed from the front. In particular, as understood from FIG. 2, the line representing the gray-scale transmittance characteristics to be observed when the liquid crystal is viewed in the oblique direction undulates to a large extent in comparison with the line representing the gray-scale transmittance characteristics to be observed when the liquid crystal display device is viewed from the front. Accordingly, when middle gray-scales are displayed, the difference in brightness becomes smaller between the viewing from the front and the viewing in the oblique direction. For this reason, a phenomenon occurs in which an image to be viewed in the oblique direction looks whitish (washes out) in comparison with that to be viewed from the front, thus deteriorating the display quality. Moreover, an anisotropy in terms of a refractive index of the liquid crystal has wavelength dependency. For this reason, color to be seen when the conventional MVA mode liquid crystal display device is viewed from the front is much different from that to be seen when the conventional MVA mode liquid crystal display device is viewed in the oblique direction in some cases.
Furthermore, the slits 15a of the picture element electrode 15 as shown in FIG. 1 are formed by use of a photolithography technique. Unevenness of the thickness of a photoresist film and a slight difference (shot irregularity) in exposure during stepper exposure make the widths of the respective slits 15a ununiformed. This causes optical characteristics of the picture element to be irregular, thus constituting a cause of display unevenness. For example, when a display is performed with middle gray scales in the entire surface of the panel, tile-shaped patterns appear in some cases.
Additionally, improvement of the substantial aperture ratio and further reduction in power consumption have been awaited. In addition, in the case of a recent liquid crystal display device, further improvement in its response characteristics has been awaited.